For example, my thought process on the whole thing was 1/4 wavelength spacing simplifies the phased array calculations, what is 1/4 wavelength at 40KHz, and the answer is, of course, it depends ...

This is where the work by Kevin C. Zhou and colleagues seeks to tick all the boxes, with a recent paper (preprint, open access) in Nature Photonics describing their 3D-RAPID system.

Array Labs is planning on scanning Earth in 3D, data that can be used for autonomous vehicle development, augmented reality and more.